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• W2084359828 abstract "Arguing against the Proposition is Mark H. Phillips, Ph.D. Dr. Phillips obtained his Ph.D. in Atomic Physics from the University of Wisconsin, Madison and his Medical Physics education at Harvard University, Boston, MA. Since 1991 he has been at the University of Washington, Seattle, where currently he is Professor in the Department of Radiation Oncology. He is certified in Radiation Oncology Physics by the American Board of Radiology. His major research interests include IMRT optimization, decision theory application in treatment planning, and applications of PET in radiotherapy. The holy grail of modern radiation therapy (RT) is to produce and efficiently deliver highly conformal dose distributions. IMRT was developed to meet this challenge and has had a significant impact on radiation oncology practice. In general, the quality of IMRT depends on beam configuration and intensity modulation. Limited by the available dose optimization and delivery techniques, little effort has been devoted to investigate systematically the role of beam angular sampling and the interplay between this and intensity modulation. Two technical advances in RT have been made recently, which are changing the current RT landscape and making a new type of treatment scheme, coined DASSIM-RT, possible.1 First, in treatment planning, a compressed-sensing based inverse planning strategy has been proposed,2 which allows the user to optimally control the level of intensity modulation of the incident beams. Second, in treatment delivery, a new generation of digital linacs with autofield sequencing has become commercially available, which dramatically improves the delivery efficiency. The clinical need for DASSIM-RT stems from the fact that conventional IMRT (with 5–10 beams) often does not possess sufficient angular sampling required to spatially spread the dose.1 In contrast, current VMAT (with 1–3 arcs) oversamples the angular space and does not provide the desired intrabeam modulation in some or all directions. Switching beam energy between the gantry angles is impossible in rotational arc delivery. DASSIM-RT explores a large area of uncharted territory in terms of the number of beams (including noncoplanar and/or nonisocentric beams) and level of intensity modulation, and bridges the gap between IMRT and VMAT. Technically, DASSIM-RT is achieved by increasing angular beam sampling while eliminating dispensable segments of the incident fields through the use of emerging compressed-sensing based dose optimization.2,3 The removal of dispensable intrabeam modulation and autofield sequencing make DASSIM-RT extremely efficient in delivery. A number of variants of DASSIM-RT are possible, such as segment boosted arc therapy in which segmented delivery at some fixed gantry angles and rotational arc delivery are intertwined to achieve a much improved dose distribution without relying on the use of multiple arcs. Of course, the boosting segments at a gantry position can also be distributed over a small angular interval and delivered rotationally by slowing the gantry rotation. In summary, DASSIM-RT is likely to replace conventional IMRT and VMAT for delivery of highly conformal radiotherapy. It represents a truly optimal RT scheme with (1) uncompromised beam sampling, (2) beam collimation,4 couch rotation5 and/or energy modulation, (3) elimination of dispensable intensity modulation, and (4) highly efficient delivery. DASSIM-RT overcomes the limitations of existing treatment schemes and empowers the radiation oncology community with the best possible tools for the next-generation of conformal RT. First, I would like to congratulate Drs. Li and Xing for their work on DASSIM-RT.1 Similar to the search for the Higgs boson, they have confirmed what many suspected, which is that there exists a multidimensional space of plans that fills the gap between VMAT and conventional IMRT plans. I believe that there will be pushback from administrators who have already invested hundreds of thousands of dollars in getting VMAT up and running. They will not be amenable to requests for purchasing new licenses for planning and delivery. Do the differences in speed between VMAT, IMRT, and DASSIM-RT really matter? Clinically, not in many cases. While intrafraction motion occurs for some tumor sites, the slow type of drift that is of interest in this comparison may not provide any operational differences between these methods with respect to the need for reimaging.6 Faster motions, i.e., respiration-induced, create the same problems for all methods and, in fact, fixed beam methods are more amenable to gating. The speed is more of an administrative/clinic issue. As the examples show, the time differences between DASSIM-RT and IMRT plans are only about a minute, and 2–3 minutes longer than a comparable VMAT plan. This seems a small effect on which to base treatment decisions. Is there a clinical benefit to the dosimetric differences? The published examples are not definitive and the differences are not dramatic.1 Strictly speaking, none of the methods dominates the others,7 although overall the advantage goes to DASSIM-RT. The move to VMAT was not instigated by better plan quality, nor is this likely to be the case with DASSIM-RT. What makes a greater difference are the optimization objectives, including the functional form and parameters.8,9 Current methods of evaluating plans are still crude, and comparisons between plans even more so. Until more comprehensive models are developed, it is very difficult to convert the array of dosimetric differences of the magnitudes reported into anything approaching significant clinical outcomes. Clinics are used to accepting plans that are “good enough” and, once a program is established, they do little exploration of plan space. Current planning systems do not provide good tools for doing so and this would require a lot of time and effort on the part of the user. The advantages of DASSIM-RT, therefore, which I am convinced are real, are not dramatic enough given the current state of treatment planning for any sizeable shift away from whatever system in which a clinic has invested. Without some other source of pressure, the advantages that physicists perceive will not yield any winds of change. I like the Higgs boson analogy but, for radiation oncology, I want to emphasize that proving what we suspected is not, should not, and will not be the end of story. Instead, it is only the beginning of a new digital RT age. Let us not forget that it took more than a decade for IMRT to go from conception to clinics worldwide. VMAT has taken even longer, apparently for the reasons rightfully listed by Dr. Phillips. Not that I am overly optimistic about technology transfer, which may have multiple causes of failure even for a totally sensible technology, but I am passionate about DASSIM-RT because of its enormous potential. Although not widely realized, radiation therapy is stepping into a digital era in which treatments will be done “station by station” instead of “beam by beam,” In a nutshell, a station (alternatively, a control point or a node) describes the state of a delivery system (including linac configurations such as beam energy, aperture shape and weight, gantry/collimator angle, and auxiliaries such as the couch). When the auxiliary equipment is stationary, a station is no different from an MLC or jaw-shaped beam. A conventional intensity-modulated beam consists of a collection of stations with the same gantry angle but different MLC segments. Next-generation RT will be all about the optimization of station-mediated intensity and spatial distribution, which I call station parameter optimized RT (SPORT). VMAT and IMRT are simply two special, and often nonoptimal, cases of SPORT, as explained in my Opening Statement. DASSIM-RT represents an important region in the SPORT map.1 The functional form and parameters of the optimization objectives do make a difference in inverse planning, as they define the solution space.8–11 SPORT also enlarges the solution space through improved angular and intensity sampling of the stations.1 About current plan evaluation methods, I rebut that they capture the main features of treatment plans and are thus useful clinically. At the bare minimum, the evaluation is like choreography—people can tell the difference when SPORT and conventional plans are placed side-by-side. That is one of the reasons that VMAT has prevailed over IMRT in the past few years. To recapitulate, SPORT/DASSIM-RT advances RT to a new paradigm through optimal modulations of station-mediated parameters. The new planning and delivery techniques will replace the existing IMRT/VMAT. Dr. Xing believes that DASSIM-RT will replace conventional IMRT and VMAT because new developments (compressed sensing and autosequencing linacs) will provide users with what they desire (efficient delivery of highly conformal dose distributions). His subsequent description of the essentials of DASSIM-RT is very convincing to mathematical physicists. But is it convincing to the physicians and administrators? As nearly all who have delved into such comparisons between methods will attest,3 the differences in the resulting plans (a) are often small and not consistent, and (b) are dependent on the skills of the particular planner. Similarly, the results presented in the original paper1 are within the variations in plan metrics that physicians see in plans produced for different patients by different planners. The delivery efficiency is only as good as or worse than that of VMAT, which does little to open the wallets of the administrators. I would be more convinced of DASSIM-RT's future if there were reliable methods to improve inverse planning. In that way, the stochastic nature of current plan quality could be overcome and the benefits of DASSIM-RT would more clearly emerge from the noise. However, our current planning environments provide few tools for systematically searching for better objectives. In addition, the limited correspondence between optimization algorithms and clinical outcomes has placed us in the curious position where clinical trials are now written to accommodate the limitations of our planning, rather than our planning being improved to accommodate clinical trials. My conclusion is that, since many institutions have already spent significant dollars for the latest planning and delivery techniques, there will not be sufficient arguments from physicians to effect any change in current habits. Perhaps, it is like new generations of smart phones, where each generation is a bit more capable than its predecessor, but where it takes a significant step forward before any but the most devoted technophile discards the old for the new. LX wishes to thank Dr. Ruijiang Li for his contributions during the development of SPORT/DASSIM-RT. He also acknowledges useful discussions with Drs. Benjamin Fahimian, Gary Luxton, Karl Bush and Daniel Chang." @default.
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• W2084359828 title "DASSIM-RT is likely to become the method of choice over conventional IMRT and VMAT for delivery of highly conformal radiotherapy" @default.
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